I believe at one point he said he got a lot of subscribers from a cross-promotion with the Now I Know newsletter (http://nowiknow.com/). I'm not sure if he has done this with other newsletters.

For daily content, I recommend http://dailyhn.com/. It provides a list of top daily stories with automatically-generated digests. The digests are of decent quality, but I just use it as a daily top ten list.